Electron capture detectors

ABSTRACT

The invention relates to an electron capture detector having an ionisation chamber of large volume and into which a sample flows at a high flow rate, the terms &#39;&#39;&#39;&#39;large&#39;&#39;&#39;&#39; and &#39;&#39;&#39;&#39;high&#39;&#39;&#39;&#39; being relative to the hitherto accepted standards for electron capture detectors. Conveniently the ionisation chamber can be in the form of an elongated tube having a length which is an order of magnitude greater than any lateral dimension.

United States Patent 11 1 1111 3,870,888 Lovelock I Mar. 11, 1975 ELECTRON CAPTURE DETECTORS 3,255,348 6/1966 Vandcrschmidt 250/83.6 FT 3,361,908 1/1968 Petitjcan 250/8311 FT [76] Inventor: James i 3.601.609 7/1971 Yuugcr 250/83.6 Ft Bowerchalke near sallsbury 3,671,740 5/1971 Marshall CI 111. 250/836 FT Wiltshire, England [22] Flled: May 1972 Primary Examiner-Harold A. Dixon [2]] Appl. No.: 250,953 Arrorney, Agent, or FirmWoodhams, Blanchard and Flynn [30] Foreign Application Priority Data May 8, 1971 Great Britain l3877/71 July 29, 1971 Great Britain 35593/71 [57] ABSTRACT [52] US. Cl 2 0/3 250/379, 250/389, The invention relates to an electron capture detector 313/54, 313/93 having an ionisation chamber of large volume and into [Sl] Int. Cl. G01t1/16 whi h a ample flows at a high flow rate the terms i 1 Field of 250/415 MR, large and high" being relative to the hitherto ac- 4; 313/5 cepted standardsfor electron capture detectors. Con- 324/33 veniently the ionisation chamber can be in the form of an elongated tube having a length which is an order of [56] References Cited magnitude greater than any lateral dimension.

UNITED STATES PATENTS 3.009963 11/1961 Roehrig 250/83.6 FT 1 Claim, 1 Drawing Figure \ONW'NG SQUPQE 3 00m ELECTRODE V GAS U FLOW 1 I I 1 CA RRIE PLUS R (C INNER ELECTPON ELECTRODE 301 55 R POTENTIAL CURRENT SOURCE DETECTOR GAS U FLOW CARRIER PLUS ELECTPON ABSORBER POTENTIAL SOURCE \omzme SOURCE 3 CURRENT OUTER ELECTRODE DETECTOR ELECTRODE 1 ELECTRON CAPTURE DETECTORS The present invention concerns electron capture detectors.

An electron capture detector comprises an ionisation chamber containing a source of ionising radiation. Upon entry of a carrier gas possessing no affinity for electrons into the chamber, recombination of positive ions and free electrons is unlikely to take place because of the high mobility of the free electrons. Thus by applying a small potential across the chamber all ions formed by the ionising radiation can be collected. When the carrier gas contains a substance having an affinity for electrons, negative ion formation occurs which is accompanied by an observed decrease in current.

For the detection of electron capturing gases or vapours such as sulphur hexafluoride and halogenated hydrocarbons, the detector can be considered as a chamber ofvolume V into which an inert carrier gas carrying the detectable gas or vapour flows at a rate U If the rate of removal of the detectable gas or vapour by its reaction with electrons is K, then the proportion of the gas or vapour converted into negative ions is shown to be:

proportion ionised KV/(KV U) The greater the proportion of molecules of the detectable gas or vapour that is ionised the greater the decrease in detector current for a given amount of the gas or vapour so that for the maximum sensitivity it is necessary to maintain the proportion ionised as large as possible.

The above relationship suggests that for a given gas or vapour (whose molecular properties together with the fixed properties of the detector, such as the size of the source of the ionising radiation and the nature of the polarising potential i.e. DC. or pulsed, determine the value of K) an increase in the carrier gas flow rate U will decrease the proportion ionised and hence the sensitivity.

Hitherto it has been assumed that maximum sensitivity is only possible with low flow rates ofthe carrier gas, for example, flow rates of 50 ml. per minute and less.

However, the relationship also suggests that the reduction in sensitivity with an increasing flow rate can be overcome by a compensatory increase in detector volume V. As a result, with a large volume detector high gas flow rates would carry a lesser penalty in terms of reduced proportion of material ionised. At the same time, high gas flow rates will convey into the detector a greater mass flow of the detectable gas or vapour. In combination these considerations imply that a large detector will be better able to detect low concentrations of electron capturing materials than a small detector.

The accompanying drawing diagrammatically illustrates apparatus in accord with the present invention.

According to the present invention there is provided a method of using an electron capture detector which comprises introducing a sample at a high flow rate U into the detector D having an ionisation chamber C of large volume V.

The terms high and large applicable to flow rate and volume respectively are relative to the hitherto accepted standards for electron capture detectors. In this context a large volume could be a volume from 2 ccs. upwards and could be in excess of 100 ccs. A high flow rate could be a flow in excess of 50 mls. per minute, and can even exceed 3 litres per minute.

Common sources of ionising radiation employed in electron 'capture detectors are Tritium and Ni The ionising radiation range of the source is required to be at least half the maximum dimension of the chamber. In certain cases, to achieve this result, it may be necessary to distribute the source lengthwise within the chamber to ensure that all of the gas therein is subjected to the radiation. Tritium and Ni are beta emitters. Alternative radio-active sources suitable for use with large electron capture detectors are gamma ray emitters such as Thulium 170, Americium 241 and Cadmium 109; Bremsstrahlung emitters such as Promethium 147 in aluminium; and other beta ray emitters such as Krypton 85, Promethium 147, Thallium 204 and Strontium 90. I

Compounds such as SP and CCl which are strong electron absorbers, are readily identified in the electron capture detector but difficulties can arise with weak electron absorbers such as oxygen and benzaldehyde. With weak absorbers only small proportions thereof may be destroyed by ionisation within the chamber and with a large volume chamber the performance of the detector may be impaired as an undesirably long time may be required to ventilate the chamber and remove the last detectable traces of the absorber.

The present invention seeks to overcome this adverse effect on performance while at the same time retaining the benefits arising from the use of a large volume chamber in the detector.

Thus according to another aspect of the present invention a method of using an electron capture detector comprises introducing a carrier gas, carrying an electron absorbing compound, into the detector ionisation chamber, the longitudinal dimension of the chamber being much greaterthan a lateral dimension.

According to yet another aspect of the present invention there is provided an electron capture detector having an elongate ionisation chamber, the length of the chamber being many times greater than a dimension normal to the length.

In this way the detector is still provided with an ionisation chamber having a large volume but the geometry of the chamber has been changed by increasing its length and decreasing its cross-section.

Examples of the advantages stemming from the use of large volume detectors are in gas chromatography and leak detection. In gas chromatography it is desirable to employ a sample of volume as large as the gas chromatographic considerations of resolution on the column, i.e. the means supplying the gas sample, and volumetric time constant at the detector will allow. This may be a sample Whose volume is between one and five times that of a detector. In leak detection, the large volume detector can permit greater rates of sample gas or vapour input from the means constituting the leak source without loss of ionising efficiency and hence a greater sensitivity to low concentrations of electron capturing gases or vapours.

In the case of a tritium detector having an elongate ionisation chamber, the chamber can be 15 cms. long with a diameter of 5 mms. These dimensions are merely examples and are not to be construed as a limitation. The minimum cross-sectional area of the chamber will be dictated by the properties of the source of ionising radiation used in the detector. Thus, when the source is Tritium a practical minimum diameter is 3 mms. It is not necessary however to adhere to cylindrical geometry and other cross-sectional configurations can be utiof 3 litres/minute from the sample source would provide the same ionisation efficiency as a detector 1 cc. in volume and a flow rate of 30 ml./minute. The sample size for the large detector can in this case be 100 times hsed- 5 greater than that of the small detector and a correln hitherto electron capture detectors where the spending improvement in Sensitivity is possible length of the ionisation chamber is of the same order I claim; of magmtude.as.the cross-Section a mlxmg of the gases 1. In an electron capture detector of the type having takes place withln the chamber. In the case of the elona detector defining an ionization chamber through gate chamber of large volume as herein proposed the which a gas sample including an electron absorber is gas or gases can be considered to move along the chamfl bl d t l d her as a discrete plug. Consequently, for a given chaman compnsmg an er ectro a ra laber volume and a given camel. gas flow rate there is a tlon source at the inner surface of said detector, and an reduction in the time required to ventilate the chammner electrode mounted wlthln Sald,detect0r an her. The elongate chamber can be considered analom t partlcularly for Improving perfrman ce gous to a plurality of short chambers coupled together with relatively weak electron absorbers and comprisin series. The proportion of an electron absorbing submg: I stance ionised in each short length of the series is the Tadloachve 'Omzmg Source dlsmbuted same but the total ionisation in the elongate chamber lengthwise through the chamber for subjecting all is the product of the ionisation in each of the individual g therein to radiation; short lengths. In this way high ionisation efficiency and an ionization chamber length which is at least ten li t i response are hi v d times greater than any lateral dimension of said Three examples of existing electron capture detecchamber for causing a gas sample to move along tors are given in the table below. The radio-active said chamber as a discrete plug, and with a minisources are arranged at the inner surface of the detecmum of mixing between adjacent sample portions, tor bodies which are the cathodes and define the ioniin which the chamber length is 15 cm. and the zation chamber. The anodes are metal rods mounted chamber diameter is 5 mm; coaxially within the detector bodies. means for flowing the gas sample into and along said Chamber Dimensions, cm.

Radioactive source Cathode Anode Volume and Strength, mCi Diameter Length Diameter Length ccs.

Nickel-63 15 L25 1.45 0.] 1.0 2.0 Tritium 200 0.50 L00 0.03 0.9 0.2 Argziiicium 0.0l5 0.95 1.3 0.1 [.0 0.95

It will be noted that the volume varies from source to chamber at a relatively high flow rate in excess of source. Thus an existing typical Ni detector can have 50 mls/minute to provide a large mass flow of said a volume of 2 ccs. and a typical tritium detector can detectable electron absorber, the reduction in prohave a volume of 0.2 ccs. An example has been given portion of said electron absorber converted into herein of atritium detector according to the invention negative ions occasioned by said high flow rate in which a chamber 15 cms. long and 5 mms. diameter b i ubstantially compensated by a correspondhas a chamber volume of approximately 3 ccs. This is ing large chamber volume, whereby high fl rate far in excess of the example given in the table- A N a and large chamber volume facilitate detecting low detector according to the invention can have a chamconcentrations f electron absorbers; her Volume in excess of 10 (305' whereby to tend to minimize the time required to re- AS an p if quantities of 6 in air are move last detectable traces of even relatively weak sought, a detector 100 ccs. in volume and a flow rate electron absorbers f the chamber 

1. In an electron capture detector of the type having a detector defining an ionization chamber through which a gas sample including an electron absorber is flowable, and comprising an outer electrode, a radiation source at the inner surface of said detector, and an inner electrode mounted within said detector, an improvement particularly for improving performance with relatively weak electron absorbers and comprising: a radioactive tritium ionizing source distributed lengthwise through the chamber for subjecting all gas therein to radiation; an ionization chamber length which is at least ten times greater than any lateral dimension of said chamber for causing a gas sample to move along said chamber as a discrete plug, and with a minimum of mixing between adjacent sample portions, in which the chamber length is 15 cm. and the chamber diameter is 5 mm; means for flowing the gas sample into and along said chamber at a relatively high flow rate in excess of 50 mls/minute to provide a large mass flow of said detectable electron absorber, the reduction in proportion of said electron absorber converted inTo negative ions occasioned by said high flow rate being substantially compensated by a corresponding large chamber volume, whereby high flow rate and large chamber volume facilitate detecting low concentrations of electron absorbers; whereby to tend to minimize the time required to remove last detectable traces of even relatively weak electron absorbers from the chamber. 